These types of method are generally known in the area of medical technology. They are especially used to align images which were obtained using different examination methods. The images involved can be both volume images and also projection images. For example the image of a patient that has been recorded with a computer tomography device can be placed over a further image created using magnetic resonance tomography. The combination of a fluorescence image with an image created with the aid of a computer tomograph represents a further example.
The calculation of the necessary transformation of coordinates is also referred to as registration. The presentation of the registered image data is also called fusion. The registration and the fusion can be undertaken with image data of the same or different modalities. Modality in this case is understood as the way in which the data is obtained. Image data of the same modality has especially been recorded with the same diagnosis device.
The image data of the same or different modalities can be registered the aid of orientation aids (=landmarks). These orientation aids, which are also referred to as landmarks, can be easily identifiable areas of the mapped object or additional markings (=fiducials) attached to the object.
There are also methods which are oriented to the overall structure of the mapped object. These methods include methods with visual position alignment and methods which compute the correlations between the voxels of the images to be registered as well as methods which are oriented to the surface of the mapped object. Voxels here are to be understood as picture elements of a volume image.
For the registration of image data a certain number of degrees of freedom of a transformation matrix are defined, which map each image coordinate of the one image onto an assigned image coordinate of the other image. The one image is referred to below as the model image and the other image as the reference image.
If the landmarks can be found in the image data, the transformation matrix can be computed in a simple manner by solving a linear equation system. To this extent no error-prone or long-winded optimization processes are necessary. In addition the transformation matrix can be calculated within a short time.
A disadvantage of the landmark-based calculation of the transformation matrix is that the landmarks are frequently unable to be found in the images. This leads to inaccuracies in the registration.